how much to shorten a stem for a wider bar

I appreciate this is very subjective but I'm looking at using a wider handlebar and am trying to decide how much to shorten the stem to keep the steering the same. As a rule of thumb does 1cm off the stem for an extra inch on the bar sound about right?

Hiyo,
I think that short stem / wide bars is an arm-shoulder-chest ergonomics thing and a center of gravity relative to the bike thing, more than geometry. I know people are sensitive to steering 'speed'. So I did the trigonometry in a crappy python script. If I had all alfternoon I would look into getting it to work in HTML somehow so it would be interactive in the thread(??). Also, this is flattened to 2D.

Sample output using numbers I think are common to XC setups looking for their first wide-bar experience:

I was really surprised to see how little bar length goes up as stem length comes down to retain the same leverage (what I called 'steering coefficient'). That's where the ergonomics issue comes in. Percieved steering speed and overall handling are going to take your body's position relative to the bars into account, not just force on a lever like some robot in a test rig. Wide feels good! But It looks like it slows your steering considerably, even w/ a short stem.

Personally, I feel that too short a stem can cause the rider to be more prone to OTB's. A wider bar won't keep keep your reach in front of you like a longer stem will. That said, I currently have a 70mm stem with a 750mm bar (which will get cut to 730mm soon.)

So I went from a 685 to a Havoc carbon fiber 750 bar with my original 100 stem. People told me I had to go to a shorter stem because it couldn't work. I'm 6-3, riding an xl Tracer vp. I like the ride/love the wider bars, but I have noticed a little bit of front end dive in some tight turns. I was thinking about going to an 80mm stem but I wonder about the overall stability over the tight corner dive? It's only 1/2" difference but is it worth the try/expense?

snfoilhat - I was thinking it would be useful to try something like this but also include head angle so you could also think about stem / handlebar combos to compensate for going with a longer fork. Can't help feeling your results from your script look a little suspect, am not really too sure about the maths behind this. Would be interesting to look at what other people have done on this, there must be something out there.

Dubthang - I think when going with the shorter stem approach you really need to give it some thought when buying the bike so you can get something with a longer top tube, the wider bar only gives you so much (or you go with a layback post if you're not concerned about maintaining the saddle position relative to the bb). Otherwise yes, if you just put on the short stem when your sat in the saddle I think it will screw up your weight distribution.

Rojo - I think to say "it wouldn't work" is a bit dramatic but you are missing out on some of the possible advantage of the short stem, its my understanding that the short stem makes the bike feel more stable on the descents as you're further behind the front wheel. I guess it all depends on how much of a bike geek you are, whether tinkering with these things is part of your interest in mountain biking or just a distraction. I'd certainly give it a go, I think the head angle of the tracer vp is 68 or 69 degrees so an 80 stem with a 750 bar would sound fairly standard, perhaps even a hair shorter, although that might be a bit much change in one go. Why not just buy a cheap stem and see how it feels.

Dubthang - I think when going with the shorter stem approach you really need to give it some thought when buying the bike so you can get something with a longer top tube, the wider bar only gives you so much (or you go with a layback post if you're not concerned about maintaining the saddle position relative to the bb). Otherwise yes, if you just put on the short stem when your sat in the saddle I think it will screw up your weight distribution.

Correct. I was thinking about it in the terms of riding while standing. Should have stated that.

Wow I never knew people put so much math into their cycling! I bought some wider bars and a shorter stem. I figured out the math like this bars 29.95 + stem 19.95 (plus postage) the first time I rode it I knew I got the right ones!